Titanium compounds have been widely used for organic syntheses mostly in the form of Lewis acid catalyst or nucleophilic reagent in which the ligand is replaced by a nucleophilic reagent. The former is used for aldol reaction and Michael reaction (Mukaiyama et al., Angew. Chem., Int. Ed. Engl., 16, 817 (1977)) or for asymmetric ene reaction (Mikami et al., J. Am. Chem. Soc., 112, 3949 (1990)), and the latter is used for reaction of an organotitanium to be obtained from a titanium compound and an organolithium or Grignard reagent (Reetz et al., “Organotitanium Reagents in Organic Synthesis”, Springer (1986)) or for reaction between an ester compound and a complex obtained from a titanium compound and an alkyl Grignard reagent (Corey et al., J. Am. Chem. Soc., 116, 9345 (1994)).
The catalytic use of a titanium compound for the coupling reaction of less reactive molecules has been limited. (For example, the use of a low-valence titanium compound (obtained from a titanium compound and a reducing agent) for dimerization of a carbonyl compound ((McMurry et al., Acc. Chem. Res., 7, 281 (1974)) and the use for the reaction that employs a Ziegler type reacting reagent obtained by combination of a titanium compound with a typical metal compound (Sato et al., Yuki Gosei Kagaku, 38, 234 (1980)).
In contrast with titanium, zirconium (which also belongs to Group IVa) is an extremely useful metal catalyst for organic syntheses. For example, it functions as a divalent catalyst typified by zirconocene (biscyclo-pentadienyl zirconium) for the reaction of the carbon-carbon unsaturated bond, which has a comparatively low reactivity. (Negishi et al., Yuki Gosei Kagaku, 47, 2 (1989)). A few reactions by titanocene (as a titanium compound) are known; but they are seldom superior to those by zirconocene. (E. Negishi, Comprehensive Organic Synthesis, vol. 5, 1163-1184 (1991); B. M. Trost, I. Fleming, L. A. Paquette, Eds., Pergamon Press). Moreover, zirconocene and titanocene are too expensive for industrial use.